1. Technical Field
This invention relates to optical scanning systems, and more particularly, to image derotators for use with optical scanners.
2. Discussion
Optical scanning systems have a wide variety of uses. These uses generally fall into two general categories: reading systems and reproducing systems. Reading systems interrogate an object scene and collect data. Reproducing systems reproduce information from processed data. Reading and reproducing systems utilize scanning mechanisms which direct either an outgoing light beam or an incoming light beam. Scanning mechanisms deflect light by means of reflective, refractive, or diffractive means. The path traced out by a scanning light beam is called a "scan trace" or "scan pattern". A scan trace may result from rotational, oscillatory or translational motion of optical components. Unfortunately, the motion of optical components used to create the scan trace can introduce undesirable alterations in the image produced. For example, rotational and oscillatory scanning can introduce undesirable image rotation. In addition, the optical components in a scanning system may also introduce polarization sensitivity. Polarization sensitivity results when the intensity of the light output from an optical component in the scanning system is dependent upon the polarization state of the incident light.
A number of systems have been developed to compensate for image rotation and polarization sensitivity in scanning systems. For example, image derotators are described in U.S. Pat. Nos. 4,166,959 and 4,202,597. However, in conventional image derotators the field of view is frequently very limited. In addition, image derotators commonly exhibit significant polarization sensitivity. One solution to the problem of polarization sensitivity in derotators, has been to employ scanning systems that do not introduce image rotation in the first place, thereby avoiding the necessity of an image derotator. Unfortunately, these scanning systems typically have more polarization sensitivity than image rotating scanners. In addition, the polarization sensitivity varies with scan angle.
One method of controlling the polarization sensitivity in a derotator is by increasing the number of mirrors, thereby reducing the angle of incidence of each mirror. This reduces the degree of polarization introduced by the reflection upon each mirror. However, transmission losses are increased with the increased number of reflections. Size and packaging constraints also limit the number of mirrors that may be employed. In addition, insofar as applicant is aware, zero polarization sensitivity has not been achieved in prior scanning system image derotators.